In this article we examine the factors which affect the determination of the plasma bulk parameters in space, with particular emphasis on the density determination. We make this assessment with reference to a particular instrument, the AMPTE‐UKS ion instrument, in order to be specific, but the issues raised here are likely to be encountered in the use of any space plasma instrument containing electrostatic energy analyzers or microchannel plate detectors. We have established a mathematical formalism for determining these parameters by relating the measured counts to the distribution function in terms of the geometric factor. The geometric factor is determined in the calibration of the instrument which is described in some detail. Among the factors we have considered are our calibration techniques, MCP efficiency, detector energy, and angular resolution, as well as the approximations used in our mathematical formalism. To establish confidence in our determination, we used a computer simulation to look for systematic errors in the particular characteristics of the analyzer and to verify the method of extraction of plasma parameters. We can conclude that the detector resolution is adequate for determining density for both solar wind (<5% error) and magnetosheath conditions (<6% error). The detector resolution is also adequate for measuring velocity, <1% error for the solar wind and <3% error in the magnetosheath. The detector resolution is not adequate for determining temperature in the solar wind ∼50%; however, in the magnetosheath where the thermal spread is at least as large as the acceptance angles, the resolution is adequate (<6%). In addition, we tested the influence of the geometric factor on the output bulk parameters by varying the input velocity direction over the entire polar range. The systematic error in the output parameters was less than 5% in all cases.